feat: Projektfundament mit Datenmodell, Migrationen und Seed

- Repo-Struktur für Backend, Frontend, Dokumentation und Gitea-Workflows
- FastAPI-Skeleton mit pydantic-settings und RFC-7807-artigem Fehlerformat
- Vollständiges Datenmodell (15 Tabellen) als SQLAlchemy-2.0-Modelle
- Alembic gegen die Async-Engine inklusive Erstmigration
- Idempotenter Seed für den deutschen Kategoriebaum und Standardregeln
- Endpunkte /api/health und /api/version
- pytest-Infrastruktur mit transaktionsisolierten Fixtures

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014e7t8UpmoVNMtWivY5LiSH

backend/app/core/errors.py

@@ -0,0 +1,108 @@
+"""Einheitliche Fehlerdarstellung im RFC-7807-Stil: {"detail": ..., "code": ...}."""
+
+from typing import Any
+
+from fastapi import FastAPI, Request, status
+from fastapi.exceptions import RequestValidationError
+from fastapi.responses import JSONResponse
+from starlette.exceptions import HTTPException as StarletteHTTPException
+
+# Starlette benennt die 422-Konstante gerade um – fester Wert vermeidet die Abhängigkeit.
+HTTP_422 = 422
+
+
+class AppError(Exception):
+    """Basisklasse für fachliche Fehler mit stabilem Fehlercode."""
+
+    status_code: int = status.HTTP_400_BAD_REQUEST
+    code: str = "bad_request"
+
+    def __init__(self, detail: str, *, code: str | None = None, status_code: int | None = None):
+        super().__init__(detail)
+        self.detail = detail
+        if code is not None:
+            self.code = code
+        if status_code is not None:
+            self.status_code = status_code
+
+
+class NotFoundError(AppError):
+    status_code = status.HTTP_404_NOT_FOUND
+    code = "not_found"
+
+
+class ConflictError(AppError):
+    status_code = status.HTTP_409_CONFLICT
+    code = "conflict"
+
+
+class ValidationError(AppError):
+    status_code = HTTP_422
+    code = "validation_error"
+
+
+class AuthError(AppError):
+    status_code = status.HTTP_401_UNAUTHORIZED
+    code = "unauthorized"
+
+
+def problem(
+    status_code: int, detail: str, code: str, extra: dict[str, Any] | None = None
+) -> JSONResponse:
+    body: dict[str, Any] = {"detail": detail, "code": code}
+    if extra:
+        body.update(extra)
+    return JSONResponse(status_code=status_code, content=body)
+
+
+# Zuordnung der Standard-HTTP-Statuscodes auf sprechende Fehlercodes.
+_STATUS_CODES = {
+    400: "bad_request",
+    401: "unauthorized",
+    403: "forbidden",
+    404: "not_found",
+    405: "method_not_allowed",
+    409: "conflict",
+    413: "payload_too_large",
+    415: "unsupported_media_type",
+    422: "validation_error",
+    429: "too_many_requests",
+    500: "internal_error",
+}
+
+
+def register_exception_handlers(app: FastAPI) -> None:
+    """Registriert alle Handler, damit jede Fehlerantwort dasselbe Format hat."""
+
+    @app.exception_handler(AppError)
+    async def _app_error(_: Request, exc: AppError) -> JSONResponse:
+        return problem(exc.status_code, exc.detail, exc.code)
+
+    @app.exception_handler(StarletteHTTPException)
+    async def _http_error(_: Request, exc: StarletteHTTPException) -> JSONResponse:
+        code = _STATUS_CODES.get(exc.status_code, "error")
+        detail = exc.detail if isinstance(exc.detail, str) else str(exc.detail)
+        return problem(exc.status_code, detail, code)
+
+    @app.exception_handler(RequestValidationError)
+    async def _validation_error(_: Request, exc: RequestValidationError) -> JSONResponse:
+        return problem(
+            HTTP_422,
+            "Die Anfrage enthält ungültige Felder.",
+            "validation_error",
+            {"errors": _serialise_errors(exc.errors())},
+        )
+
+
+def _serialise_errors(errors: list[Any]) -> list[dict[str, Any]]:
+    """Pydantic-Fehler auf JSON-serialisierbare Felder reduzieren."""
+    result = []
+    for error in errors:
+        result.append(
+            {
+                "loc": [str(part) for part in error.get("loc", [])],
+                "msg": error.get("msg", ""),
+                "type": error.get("type", ""),
+            }
+        )
+    return result